Fair enough so let's take your own list:
Money - we can offer better wages without doubt
Location - London or Southampton mmmmm...
Career progression - Our recent high profile sells have been to the likes of Real Madrid...
We also have no genuine defensive midfielder so no real competition for his place.
Get my drift?
Money - Southampton simply value him more than we do. They will make him the highest paid player at the club... hell, they will break their standard wage structure and offer him 80k / week in order to land him... they value him that much. Meanwhile Spurs see him as fourth choice on their wish-list for that position and wouldn't be willing to offer more than 50k because
that's how much we value him. The notion that every team will make an identical assessment of the value of the player and are simply willing to offer their maximum wage every time there's a tiny bit of competition for a players signature is a bizarrely simplistic idea. And it's demonstrably false.
Location - He hates big capital cities. Southampton is a much better fit for him than London. Just like Marseilles it's a south-coast town... big enough but not too big. Also, his best mate is from Southampton and he spent several months there when he was a kid. Absolutely loves the place.
Career progression - He believes that being the main man, who is absolutely guaranteed to be the first name on the team-sheet, will showcase him better, even if Southampton is a smaller team. On top of which, he's a massive fan of Koeman and has wanted to work with him all his life.
Get my drift?
Is any of the above true? I have no idea... almost certainly most of it isn't. But it's frustrating to see otherwise intelligent people insist that "we can offer better wages" because it suits their argument. I just don't buy it, Maltese Falcon, that you honestly believe every club values every player at exactly the same level and are prepared to offer their highest wage to everyone - regardless of that internal valuation.
That stuff about not liking big cities; his mate from Southampton; his respect for Koeman. Sure, I made all that up. The point was to demonstrate that unless you are an honest-to-GHod ITK when it comes to Imbula, there is likely to be a shed-load of stuff you simply don't know about him that might affect his decision.